1. Field of the Invention
The present invention relates to imaging systems. More specifically, the present invention relates to radar imaging systems.
2. Description of the Related Art
Imaging techniques are well known and widely used in the art. Certain imaging technologies are better suited for particular applications. For example, radar imagery is widely used for surveillance and reconnaissance as well as target tracking and identification. For radar and other imaging technologies, the ability to clearly resolve and discriminate targets may be essential in meeting objectives specified for a particular application.
One such application involves xe2x80x98real beam ground mappingxe2x80x99. Real beam ground mapping involves scanning an area, e.g., the earth""s surface, using a scanning antenna or an electronically scanned antenna. Returns from an illumination of the surface are then examined for xe2x80x98back-scatterxe2x80x99 or reflections therefrom. As the beam is scanned in azimuth, information is collected with respect to the range direction. At each beam position, the distance of various scatterers may be ascertained for each range cell. This information may then be displayed in a real beam ground mapped image.
Unfortunately, while range data may be resolved with adequate resolution, currently, resolution of azimuth data with comparable resolution has proved to be problematic. This is due to the fact that azimuth resolution is limited to the width of the beam and degrades as a function of range. Accordingly, the poor resolution of conventional real beam mapping systems limits the ability of the system to discriminate scatterers.
SAR (synthetic aperture radar) has been used for ground mapping. However, currently, SARs require several seconds at each beam position and are therefore too slow for many more demanding (e.g., military) applications.
xe2x80x9cSuper resolutionxe2x80x9d techniques are widely used to sharpen the radar imagery. However, the quality achieved is scene dependent and is not robust. The current techniques do not effectively account for the impact of the radar system on the true scene.
Hence, a need remains in the art for an improved system or method for providing ground-mapped images. Specifically, a need remains in the art for a system or method for providing enhanced cross-range (azimuthal) resolution for a real beam ground mapping radar system.
The need in the art is addressed by the image processing system and method of the present invention. In an illustrative embodiment, the inventive system is implemented in software running on a processor. The software provides a transfer function between scene reflectivities and image intensities in image databased on geometry, beam pattern and/or scan rate. The software ascertains a set of scene intensities that minimize a penalty function of the transfer function. Further, the software ascertains an optimal set of weights for the scene reflectivities based on the weighted scene intensities for the image data.
In an illustrative application, the invention is incorporated into a radar system including: a transmitter for illuminating a target with a beam of electromagnetic energy, the beam scanning in a cross-range direction; a receiver for receiving reflections of the beam as return data and convolving the beam with the return data; and a processor responsive to the receiver for enhancing the target return in the cross-range direction.
In short, the inventive method provides a novel technique, which provides significantly enhanced image sharpening. In the illustrative embodiment, the inventive method uses an iterative convergence technique that minimizes a penalty function whose weights are updated at each stage of the iteration. The present invention provides a technique for significantly enhancing radar imagery by iteratively deriving a best scene solution that reduces corruption introduced by a radar system. The inventive technique models the true scene signal corruption and derives a solution for the scene intensities, which minimizes the errors in the derived image.
The novel technique finds the scene scatterer powers, which best match the original image pixel powers. The effect of the antenna pattern is taken into consideration when computing the derived image, which is matched against the original image. For example, for Real Beam Ground Maps the power PDId (post detection noncoherently integrated) pixel responses are considered with the given known smearing of the antenna. In the illustrative embodiment, the penalty function is the sum of square errors between the hypothesized scene excitation convolved with the antenna pattern and the radar voltage returns. The weights are adjusted on each iteration based on the updated estimates of the scene intensities.